1. Field of the Invention
The present invention relates to a shock absorbing structure for a vehicle. More particularly, the invention relates to a shock absorbing structure for a vehicle that reduces a shock applied to a pedestrian and a vehicle passenger at the time of a contact or collision with the pedestrian or other objects, to thereby protect the pedestrian and the passenger.
2. Description of the Related Art
Conventionally, in many vehicles such as automobiles, a shock absorbing structure is disposed at a side opposite from a vehicle cabin side (a back side) of an interior component such as a pillar garnish, a roof side rail, or an instrumental panel that is more likely to contact with the head or the leg of a driver or a passenger in a vehicle collision, or disposed inside an exterior component such as a bumper that is more likely to contact with a pedestrian. Thus, the shock absorbing structure can protect a vehicle passenger, a pedestrian or the like by reducing a shock applied to them when they contact with the interior or the exterior component at the time of a collision or the like.
Various types of shock absorbing structures are conventionally well known. As one type of such a shock absorbing structure for a vehicle, there is known a shock absorbing structure (a) comprising a resin molded body having an angular U-shaped cross section parallel to a shock application direction. The resin molded body includes a top wall to which a shock is applied and two side walls formed integrally with the top wall such that the side walls extend in a shock application direction from a back surface of the top wall opposite to a surface on which the shock is applied, while being opposed to each other. The side walls are deformed by application of shock, thereby absorbing the shock (See JP-A-2005-104164, for example). Since the shock absorbing structure for a vehicle is made of the resin molded body, excellent moldability and weight reduction can be obtained. In addition, ideal load displacement characteristics represented by a rectangular waveform can be obtained by absorbing the shock via buckling deformation of the side walls.
However, in order to increase an absorbing amount of shock energy in a limited shock stroke in accordance with a size of an installation space for the structure, the above shock absorbing structure (a) generally employs a structure in which a load value in the load displacement characteristics is increased within an allowable range by allowing thickness of the side walls to be larger, for example. However, in fact, it is difficult to increase the absorbing amount of the shock energy to an expected level only by allowing the side walls to have a larger thickness. Using such thick side walls will cause an increase in weight of the shock absorbing structure. On the other hand, if the load value in the load displacement characteristics is adjusted by allowing the side walls to have a small thickness, a large scale modification is needed to modify a core or an entire surface of a cavity surface of the mold die of the shock absorbing structure that is made of the resin molded body, thus leading to a production cost increase. In short, it is not easy for the shock absorbing structure (a) that is merely made of the resin molded body having the angular U-shaped cross section to tune the load displacement characteristics such that the absorbing amount of shock energy is set to a desired amount without any increase in weight, cost, and the like.
Under such circumstances, there is proposed a shock absorbing structure for a vehicle (b) that includes a top wall and two side walls integrally formed with the top wall so as to have a basic configuration with an angular U-shaped cross section like the above conventional shock absorbing structure, and each side wall has a waveform in which convex portions protruded outwardly in opposing directions of the side walls, i.e., in directions in which the side walls are opposed to each other, and concave portions recessed inwardly in the opposing directions of the side walls are alternately and continuously arranged in a length direction of the side walls (See JP-A-2001-354092, for example). Additionally, there is proposed a shock absorbing structure for a vehicle (c) that includes a top wall; side walls having not only a waveform but also having windows and slits that are extended in a shock application direction and that are formed in protruded portions of convex portions, in bottom portions of concave portions, or in opposite side portions of the concave portions of the side walls; and a bottom plate having an outer flange-like shape that is extended outwardly in the opposing directions of the side walls and that is integrally formed with end portions of the side walls opposite from the top wall side so as to be extended over an entire length in a length direction perpendicular to the shock application direction (See U.S. Pat. No. 6,726,262 and US 2007200375, for example).
Among the conventional shock absorbing structures as above, in the shock absorbing structure (b), the side walls are waveform shaped, so that even when lengths of the side walls extended in the shock application direction are made small, the load value in the load displacement characteristics can be made sufficiently large. Moreover, strengths of the side walls can be changed, without changing the thickness of the side walls at all, by variously changing heights and depths of the convex and the concave portions of the waveform-shaped side walls. This enables the load value in the load displacement characteristics to be favorably increased or reduced. However, in order to change the heights and the depths of the convex and the concave portions of the side walls, as in the change of the thickness of the side walls, it is necessary to provide a large scale modification to the core or the entire cavity surface of a mold die. Thus, also in the shock absorbing structure (b), production cost inevitably increases due to tuning of the load displacement characteristics.
In the shock absorbing structure (c), the load value in the load displacement characteristics can be favorably increased or reduced by adjusting the size of the windows, the width and the length of the slits, and the like provided in the protruded portions of the convex portions, the bottom portions of the concave portions, or the opposite side portions of the concave portions of the side walls. In this case, there is no need to change the thickness of the side walls, the heights and the depths of the convex and the concave portions of the side walls, or the like at all. In addition, the bottom plate connects the convex portions adjacent to each other on the side walls, and also connects the side portions of each of the concave portions to each other. Thus, it can be favorably prevented that deformation strength of the entire side walls is excessively reduced by formation of the windows and the slits in the convex and the concave portions. Accordingly, unlike the case in which the thickness of the side walls, the heights and the depths of the convex and the concave portions of the side walls, or the like are changed, only a small scale modification is required to the mold die, such as a modification to only a part of the cavity surface which provides the windows and the slits, in order to increase or reduce the load value in the load displacement characteristics. Thereby, the load displacement characteristics can be surely tuned at the lowest possible cost.
However, in the shock absorbing structure (c) including the side walls having the waveform, although the above advantages can be obtained, a distance between the bottom portions of the concave portions positioned on opposite sides in the mutually opposing directions of the side walls is inevitably made small. Accordingly, when the side walls are buckled and deformed by application of a shock, the bottom portions of the concave portions can easily contact with each other. This causes a risk that the shock stroke of the side walls may be insufficient. If that happens, the absorbing amount of shock energy cannot be ensured sufficiently, even though the sizes of the slits and the windows are adjusted to tune the load displacement characteristics.